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libs/corosio/src/corosio/src/detail/select/scheduler.cpp

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libs/corosio/src/corosio/src/detail/select/scheduler.cpp
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1 //
2 // Copyright (c) 2026 Steve Gerbino
3 //
4 // Distributed under the Boost Software License, Version 1.0. (See accompanying
5 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
6 //
7 // Official repository: https://github.com/cppalliance/corosio
8 //
9
10 #include <boost/corosio/detail/platform.hpp>
11
12 #if BOOST_COROSIO_HAS_SELECT
13
14 #include "src/detail/select/scheduler.hpp"
15 #include "src/detail/select/op.hpp"
16 #include "src/detail/make_err.hpp"
17 #include "src/detail/posix/resolver_service.hpp"
18 #include "src/detail/posix/signals.hpp"
19
20 #include <boost/corosio/detail/except.hpp>
21 #include <boost/corosio/detail/thread_local_ptr.hpp>
22
23 #include <chrono>
24 #include <limits>
25
26 #include <errno.h>
27 #include <fcntl.h>
28 #include <sys/select.h>
29 #include <sys/socket.h>
30 #include <unistd.h>
31
32 /*
33 select Scheduler - Single Reactor Model
34 =======================================
35
36 This scheduler mirrors the epoll_scheduler design but uses select() instead
37 of epoll for I/O multiplexing. The thread coordination strategy is identical:
38 one thread becomes the "reactor" while others wait on a condition variable.
39
40 Thread Model
41 ------------
42 - ONE thread runs select() at a time (the reactor thread)
43 - OTHER threads wait on wakeup_event_ (condition variable) for handlers
44 - When work is posted, exactly one waiting thread wakes via notify_one()
45
46 Key Differences from epoll
47 --------------------------
48 - Uses self-pipe instead of eventfd for interruption (more portable)
49 - fd_set rebuilding each iteration (O(n) vs O(1) for epoll)
50 - FD_SETSIZE limit (~1024 fds on most systems)
51 - Level-triggered only (no edge-triggered mode)
52
53 Self-Pipe Pattern
54 -----------------
55 To interrupt a blocking select() call (e.g., when work is posted or a timer
56 expires), we write a byte to pipe_fds_[1]. The read end pipe_fds_[0] is
57 always in the read_fds set, so select() returns immediately. We drain the
58 pipe to clear the readable state.
59
60 fd-to-op Mapping
61 ----------------
62 We use an unordered_map<int, fd_state> to track which operations are
63 registered for each fd. This allows O(1) lookup when select() returns
64 ready fds. Each fd can have at most one read op and one write op registered.
65 */
66
67 namespace boost::corosio::detail {
68
69 namespace {
70
71 struct scheduler_context
72 {
73 select_scheduler const* key;
74 scheduler_context* next;
75 };
76
77 corosio::detail::thread_local_ptr<scheduler_context> context_stack;
78
79 struct thread_context_guard
80 {
81 scheduler_context frame_;
82
83 95 explicit thread_context_guard(
84 select_scheduler const* ctx) noexcept
85 95 : frame_{ctx, context_stack.get()}
86 {
87 95 context_stack.set(&frame_);
88 95 }
89
90 95 ~thread_context_guard() noexcept
91 {
92 95 context_stack.set(frame_.next);
93 95 }
94 };
95
96 } // namespace
97
98 120 select_scheduler::
99 select_scheduler(
100 capy::execution_context& ctx,
101 120 int)
102 120 : pipe_fds_{-1, -1}
103 120 , outstanding_work_(0)
104 120 , stopped_(false)
105 120 , shutdown_(false)
106 120 , max_fd_(-1)
107 120 , reactor_running_(false)
108 120 , reactor_interrupted_(false)
109 240 , idle_thread_count_(0)
110 {
111 // Create self-pipe for interrupting select()
112
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 120 if (::pipe(pipe_fds_) < 0)
113 ✗ detail::throw_system_error(make_err(errno), "pipe");
114
115 // Set both ends to non-blocking and close-on-exec
116
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 360 for (int i = 0; i < 2; ++i)
117 {
118
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 240 int flags = ::fcntl(pipe_fds_[i], F_GETFL, 0);
119
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 240 if (flags == -1)
120 {
121 ✗ int errn = errno;
122 ✗ ::close(pipe_fds_[0]);
123 ✗ ::close(pipe_fds_[1]);
124 ✗ detail::throw_system_error(make_err(errn), "fcntl F_GETFL");
125 }
126
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 240 if (::fcntl(pipe_fds_[i], F_SETFL, flags | O_NONBLOCK) == -1)
127 {
128 ✗ int errn = errno;
129 ✗ ::close(pipe_fds_[0]);
130 ✗ ::close(pipe_fds_[1]);
131 ✗ detail::throw_system_error(make_err(errn), "fcntl F_SETFL");
132 }
133
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 240 if (::fcntl(pipe_fds_[i], F_SETFD, FD_CLOEXEC) == -1)
134 {
135 ✗ int errn = errno;
136 ✗ ::close(pipe_fds_[0]);
137 ✗ ::close(pipe_fds_[1]);
138 ✗ detail::throw_system_error(make_err(errn), "fcntl F_SETFD");
139 }
140 }
141
142
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 120 timer_svc_ = &get_timer_service(ctx, *this);
143
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 120 timer_svc_->set_on_earliest_changed(
144 timer_service::callback(
145 this,
146 3623 [](void* p) { static_cast<select_scheduler*>(p)->interrupt_reactor(); }));
147
148 // Initialize resolver service
149
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 120 get_resolver_service(ctx, *this);
150
151 // Initialize signal service
152
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 120 get_signal_service(ctx, *this);
153
154 // Push task sentinel to interleave reactor runs with handler execution
155 120 completed_ops_.push(&task_op_);
156 120 }
157
158 240 select_scheduler::
159 120 ~select_scheduler()
160 {
161
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 120 if (pipe_fds_[0] >= 0)
162 120 ::close(pipe_fds_[0]);
163
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 120 if (pipe_fds_[1] >= 0)
164 120 ::close(pipe_fds_[1]);
165 240 }
166
167 void
168 120 select_scheduler::
169 shutdown()
170 {
171 {
172
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 120 std::unique_lock lock(mutex_);
173 120 shutdown_ = true;
174
175
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 240 while (auto* h = completed_ops_.pop())
176 {
177
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 120 if (h == &task_op_)
178 120 continue;
179 ✗ lock.unlock();
180 ✗ h->destroy();
181 ✗ lock.lock();
182 120 }
183 120 }
184
185 120 outstanding_work_.store(0, std::memory_order_release);
186
187
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 120 if (pipe_fds_[1] >= 0)
188 120 interrupt_reactor();
189
190 120 wakeup_event_.notify_all();
191 120 }
192
193 void
194 3832 select_scheduler::
195 post(capy::coro h) const
196 {
197 struct post_handler final
198 : scheduler_op
199 {
200 capy::coro h_;
201
202 explicit
203 3832 post_handler(capy::coro h)
204 3832 : h_(h)
205 {
206 3832 }
207
208 7664 ~post_handler() = default;
209
210 3832 void operator()() override
211 {
212 3832 auto h = h_;
213
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 3832 delete this;
214
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 3832 h.resume();
215 3832 }
216
217 ✗ void destroy() override
218 {
219 ✗ delete this;
220 ✗ }
221 };
222
223
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 3832 auto ph = std::make_unique<post_handler>(h);
224 3832 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
225
226
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 3832 std::unique_lock lock(mutex_);
227 3832 completed_ops_.push(ph.release());
228
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 3832 wake_one_thread_and_unlock(lock);
229 3832 }
230
231 void
232 231525 select_scheduler::
233 post(scheduler_op* h) const
234 {
235 231525 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
236
237
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 231525 std::unique_lock lock(mutex_);
238 231525 completed_ops_.push(h);
239
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 231525 wake_one_thread_and_unlock(lock);
240 231525 }
241
242 void
243 4037 select_scheduler::
244 on_work_started() noexcept
245 {
246 4037 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
247 4037 }
248
249 void
250 4031 select_scheduler::
251 on_work_finished() noexcept
252 {
253
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 8062 if (outstanding_work_.fetch_sub(1, std::memory_order_acq_rel) == 1)
254 ✗ stop();
255 4031 }
256
257 bool
258 235214 select_scheduler::
259 running_in_this_thread() const noexcept
260 {
261
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 235214 for (auto* c = context_stack.get(); c != nullptr; c = c->next)
262
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 235059 if (c->key == this)
263 235059 return true;
264 155 return false;
265 }
266
267 void
268 1 select_scheduler::
269 stop()
270 {
271 1 bool expected = false;
272
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 1 if (stopped_.compare_exchange_strong(expected, true,
273 std::memory_order_release, std::memory_order_relaxed))
274 {
275 // Wake all threads so they notice stopped_ and exit
276 {
277
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 1 std::lock_guard lock(mutex_);
278 1 wakeup_event_.notify_all();
279 1 }
280
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 1 interrupt_reactor();
281 }
282 1 }
283
284 bool
285 1 select_scheduler::
286 stopped() const noexcept
287 {
288 1 return stopped_.load(std::memory_order_acquire);
289 }
290
291 void
292 34 select_scheduler::
293 restart()
294 {
295 34 stopped_.store(false, std::memory_order_release);
296 34 }
297
298 std::size_t
299 89 select_scheduler::
300 run()
301 {
302
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 89 if (stopped_.load(std::memory_order_acquire))
303 ✗ return 0;
304
305
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 178 if (outstanding_work_.load(std::memory_order_acquire) == 0)
306 {
307 ✗ stop();
308 ✗ return 0;
309 }
310
311 89 thread_context_guard ctx(this);
312
313 89 std::size_t n = 0;
314
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 242367 while (do_one(-1))
315
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 242278 if (n != (std::numeric_limits<std::size_t>::max)())
316 242278 ++n;
317 89 return n;
318 89 }
319
320 std::size_t
321 ✗ select_scheduler::
322 run_one()
323 {
324 ✗ if (stopped_.load(std::memory_order_acquire))
325 ✗ return 0;
326
327 ✗ if (outstanding_work_.load(std::memory_order_acquire) == 0)
328 {
329 ✗ stop();
330 ✗ return 0;
331 }
332
333 ✗ thread_context_guard ctx(this);
334 ✗ return do_one(-1);
335 ✗ }
336
337 std::size_t
338 7 select_scheduler::
339 wait_one(long usec)
340 {
341
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 7 if (stopped_.load(std::memory_order_acquire))
342 ✗ return 0;
343
344
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 14 if (outstanding_work_.load(std::memory_order_acquire) == 0)
345 {
346
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 1 stop();
347 1 return 0;
348 }
349
350 6 thread_context_guard ctx(this);
351
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 6 return do_one(usec);
352 6 }
353
354 std::size_t
355 ✗ select_scheduler::
356 poll()
357 {
358 ✗ if (stopped_.load(std::memory_order_acquire))
359 ✗ return 0;
360
361 ✗ if (outstanding_work_.load(std::memory_order_acquire) == 0)
362 {
363 ✗ stop();
364 ✗ return 0;
365 }
366
367 ✗ thread_context_guard ctx(this);
368
369 ✗ std::size_t n = 0;
370 ✗ while (do_one(0))
371 ✗ if (n != (std::numeric_limits<std::size_t>::max)())
372 ✗ ++n;
373 ✗ return n;
374 ✗ }
375
376 std::size_t
377 ✗ select_scheduler::
378 poll_one()
379 {
380 ✗ if (stopped_.load(std::memory_order_acquire))
381 ✗ return 0;
382
383 ✗ if (outstanding_work_.load(std::memory_order_acquire) == 0)
384 {
385 ✗ stop();
386 ✗ return 0;
387 }
388
389 ✗ thread_context_guard ctx(this);
390 ✗ return do_one(0);
391 ✗ }
392
393 void
394 7048 select_scheduler::
395 register_fd(int fd, select_op* op, int events) const
396 {
397 // Validate fd is within select() limits
398
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 7048 if (fd < 0 || fd >= FD_SETSIZE)
399 ✗ detail::throw_system_error(make_err(EINVAL), "select: fd out of range");
400
401 {
402
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 7048 std::lock_guard lock(mutex_);
403
404
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 7048 auto& state = registered_fds_[fd];
405
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 7048 if (events & event_read)
406 3623 state.read_op = op;
407
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 7048 if (events & event_write)
408 3425 state.write_op = op;
409
410
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 7048 if (fd > max_fd_)
411 186 max_fd_ = fd;
412 7048 }
413
414 // Wake the reactor so a thread blocked in select() rebuilds its fd_sets
415 // with the newly registered fd.
416 7048 interrupt_reactor();
417 7048 }
418
419 void
420 7022 select_scheduler::
421 deregister_fd(int fd, int events) const
422 {
423
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 7022 std::lock_guard lock(mutex_);
424
425
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 7022 auto it = registered_fds_.find(fd);
426
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 7022 if (it == registered_fds_.end())
427 6901 return;
428
429
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 121 if (events & event_read)
430 121 it->second.read_op = nullptr;
431
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 121 if (events & event_write)
432 ✗ it->second.write_op = nullptr;
433
434 // Remove entry if both are null
435
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 121 if (!it->second.read_op && !it->second.write_op)
436 {
437
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 121 registered_fds_.erase(it);
438
439 // Recalculate max_fd_ if needed
440
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 121 if (fd == max_fd_)
441 {
442 120 max_fd_ = pipe_fds_[0]; // At minimum, the pipe read end
443
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 120 for (auto& [registered_fd, state] : registered_fds_)
444 {
445 ✗ if (registered_fd > max_fd_)
446 ✗ max_fd_ = registered_fd;
447 }
448 }
449 }
450 7022 }
451
452 void
453 7048 select_scheduler::
454 work_started() const noexcept
455 {
456 7048 outstanding_work_.fetch_add(1, std::memory_order_relaxed);
457 7048 }
458
459 void
460 242411 select_scheduler::
461 work_finished() const noexcept
462 {
463
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 484822 if (outstanding_work_.fetch_sub(1, std::memory_order_acq_rel) == 1)
464 {
465 // Last work item completed - wake all threads so they can exit.
466 90 std::unique_lock lock(mutex_);
467 90 wakeup_event_.notify_all();
468
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 90 if (reactor_running_ && !reactor_interrupted_)
469 {
470 ✗ reactor_interrupted_ = true;
471 ✗ lock.unlock();
472 ✗ interrupt_reactor();
473 }
474 90 }
475 242411 }
476
477 void
478 14413 select_scheduler::
479 interrupt_reactor() const
480 {
481 14413 char byte = 1;
482
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 14413 [[maybe_unused]] auto r = ::write(pipe_fds_[1], &byte, 1);
483 14413 }
484
485 void
486 235357 select_scheduler::
487 wake_one_thread_and_unlock(std::unique_lock<std::mutex>& lock) const
488 {
489
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 235357 if (idle_thread_count_ > 0)
490 {
491 // Idle worker exists - wake it via condvar
492 ✗ wakeup_event_.notify_one();
493 ✗ lock.unlock();
494 }
495
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 235357 else if (reactor_running_ && !reactor_interrupted_)
496 {
497 // No idle workers but reactor is running - interrupt it
498 3621 reactor_interrupted_ = true;
499 3621 lock.unlock();
500 3621 interrupt_reactor();
501 }
502 else
503 {
504 // No one to wake
505 231736 lock.unlock();
506 }
507 235357 }
508
509 struct work_guard
510 {
511 select_scheduler const* self;
512 242284 ~work_guard() { self->work_finished(); }
513 };
514
515 long
516 10491 select_scheduler::
517 calculate_timeout(long requested_timeout_us) const
518 {
519
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 10491 if (requested_timeout_us == 0)
520 ✗ return 0;
521
522 10491 auto nearest = timer_svc_->nearest_expiry();
523
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 10491 if (nearest == timer_service::time_point::max())
524 35 return requested_timeout_us;
525
526 10456 auto now = std::chrono::steady_clock::now();
527
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 10456 if (nearest <= now)
528 51 return 0;
529
530
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 10405 auto timer_timeout_us = std::chrono::duration_cast<std::chrono::microseconds>(
531
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 20810 nearest - now).count();
532
533
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 10405 if (requested_timeout_us < 0)
534 10405 return static_cast<long>(timer_timeout_us);
535
536 ✗ return static_cast<long>((std::min)(
537 ✗ static_cast<long long>(requested_timeout_us),
538 ✗ static_cast<long long>(timer_timeout_us)));
539 }
540
541 void
542 129804 select_scheduler::
543 run_reactor(std::unique_lock<std::mutex>& lock)
544 {
545 // Calculate timeout considering timers, use 0 if interrupted
546
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 129804 long effective_timeout_us = reactor_interrupted_ ? 0 : calculate_timeout(-1);
547
548 // Build fd_sets from registered_fds_
549 fd_set read_fds, write_fds, except_fds;
550
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 2206668 FD_ZERO(&read_fds);
551
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 2206668 FD_ZERO(&write_fds);
552
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 2206668 FD_ZERO(&except_fds);
553
554 // Always include the interrupt pipe
555 129804 FD_SET(pipe_fds_[0], &read_fds);
556 129804 int nfds = pipe_fds_[0];
557
558 // Add registered fds
559
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 146889 for (auto& [fd, state] : registered_fds_)
560 {
561
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 17085 if (state.read_op)
562 13660 FD_SET(fd, &read_fds);
563
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 17085 if (state.write_op)
564 {
565 3425 FD_SET(fd, &write_fds);
566 // Also monitor for errors on connect operations
567 3425 FD_SET(fd, &except_fds);
568 }
569
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 17085 if (fd > nfds)
570 13664 nfds = fd;
571 }
572
573 // Convert timeout to timeval
574 struct timeval tv;
575 129804 struct timeval* tv_ptr = nullptr;
576
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 129804 if (effective_timeout_us >= 0)
577 {
578 129769 tv.tv_sec = effective_timeout_us / 1000000;
579 129769 tv.tv_usec = effective_timeout_us % 1000000;
580 129769 tv_ptr = &tv;
581 }
582
583
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 129804 lock.unlock();
584
585
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 129804 int ready = ::select(nfds + 1, &read_fds, &write_fds, &except_fds, tv_ptr);
586 129804 int saved_errno = errno;
587
588 // Process timers outside the lock
589
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 129804 timer_svc_->process_expired();
590
591
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 129804 if (ready < 0 && saved_errno != EINTR)
592 ✗ detail::throw_system_error(make_err(saved_errno), "select");
593
594 // Re-acquire lock before modifying completed_ops_
595
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 129804 lock.lock();
596
597 // Drain the interrupt pipe if readable
598
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 129804 if (ready > 0 && FD_ISSET(pipe_fds_[0], &read_fds))
599 {
600 char buf[256];
601
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 21554 while (::read(pipe_fds_[0], buf, sizeof(buf)) > 0) {}
602 }
603
604 // Process I/O completions
605 129804 int completions_queued = 0;
606
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 129804 if (ready > 0)
607 {
608 // Iterate over registered fds (copy keys to avoid iterator invalidation)
609 10777 std::vector<int> fds_to_check;
610
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 10777 fds_to_check.reserve(registered_fds_.size());
611
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 24471 for (auto& [fd, state] : registered_fds_)
612
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 13694 fds_to_check.push_back(fd);
613
614
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 24471 for (int fd : fds_to_check)
615 {
616
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 13694 auto it = registered_fds_.find(fd);
617
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 13694 if (it == registered_fds_.end())
618 ✗ continue;
619
620 13694 auto& state = it->second;
621
622 // Check for errors (especially for connect operations)
623 13694 bool has_error = FD_ISSET(fd, &except_fds);
624
625 // Process read readiness
626
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 13694 if (state.read_op && (FD_ISSET(fd, &read_fds) || has_error))
627 {
628 3502 auto* op = state.read_op;
629 // Claim the op by exchanging to unregistered. Both registering and
630 // registered states mean the op is ours to complete.
631 3502 auto prev = op->registered.exchange(
632 select_registration_state::unregistered, std::memory_order_acq_rel);
633
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 3502 if (prev != select_registration_state::unregistered)
634 {
635 3502 state.read_op = nullptr;
636
637
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 3502 if (has_error)
638 {
639 ✗ int errn = 0;
640 ✗ socklen_t len = sizeof(errn);
641 ✗ if (::getsockopt(fd, SOL_SOCKET, SO_ERROR, &errn, &len) < 0)
642 ✗ errn = errno;
643 ✗ if (errn == 0)
644 ✗ errn = EIO;
645 ✗ op->complete(errn, 0);
646 }
647 else
648 {
649 3502 op->perform_io();
650 }
651
652 3502 completed_ops_.push(op);
653 3502 ++completions_queued;
654 }
655 }
656
657 // Process write readiness
658
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 13694 if (state.write_op && (FD_ISSET(fd, &write_fds) || has_error))
659 {
660 3425 auto* op = state.write_op;
661 // Claim the op by exchanging to unregistered. Both registering and
662 // registered states mean the op is ours to complete.
663 3425 auto prev = op->registered.exchange(
664 select_registration_state::unregistered, std::memory_order_acq_rel);
665
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 3425 if (prev != select_registration_state::unregistered)
666 {
667 3425 state.write_op = nullptr;
668
669
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 3425 if (has_error)
670 {
671 ✗ int errn = 0;
672 ✗ socklen_t len = sizeof(errn);
673 ✗ if (::getsockopt(fd, SOL_SOCKET, SO_ERROR, &errn, &len) < 0)
674 ✗ errn = errno;
675 ✗ if (errn == 0)
676 ✗ errn = EIO;
677 ✗ op->complete(errn, 0);
678 }
679 else
680 {
681 3425 op->perform_io();
682 }
683
684 3425 completed_ops_.push(op);
685 3425 ++completions_queued;
686 }
687 }
688
689 // Clean up empty entries
690
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 13694 if (!state.read_op && !state.write_op)
691
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 6927 registered_fds_.erase(it);
692 }
693 10777 }
694
695
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 129804 if (completions_queued > 0)
696 {
697
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 3506 if (completions_queued == 1)
698 85 wakeup_event_.notify_one();
699 else
700 3421 wakeup_event_.notify_all();
701 }
702 129804 }
703
704 std::size_t
705 242373 select_scheduler::
706 do_one(long timeout_us)
707 {
708
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 242373 std::unique_lock lock(mutex_);
709
710 for (;;)
711 {
712
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 372177 if (stopped_.load(std::memory_order_acquire))
713 ✗ return 0;
714
715 372177 scheduler_op* op = completed_ops_.pop();
716
717
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 372177 if (op == &task_op_)
718 {
719 129893 bool more_handlers = !completed_ops_.empty();
720
721
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 129893 if (!more_handlers)
722 {
723
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 21160 if (outstanding_work_.load(std::memory_order_acquire) == 0)
724 {
725 89 completed_ops_.push(&task_op_);
726 89 return 0;
727 }
728
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 10491 if (timeout_us == 0)
729 {
730 ✗ completed_ops_.push(&task_op_);
731 ✗ return 0;
732 }
733 }
734
735
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 129804 reactor_interrupted_ = more_handlers || timeout_us == 0;
736 129804 reactor_running_ = true;
737
738
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 129804 if (more_handlers && idle_thread_count_ > 0)
739 ✗ wakeup_event_.notify_one();
740
741
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 129804 run_reactor(lock);
742
743 129804 reactor_running_ = false;
744 129804 completed_ops_.push(&task_op_);
745 129804 continue;
746 129804 }
747
748
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 242284 if (op != nullptr)
749 {
750
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 242284 lock.unlock();
751 242284 work_guard g{this};
752
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 242284 (*op)();
753 242284 return 1;
754 242284 }
755
756 ✗ if (outstanding_work_.load(std::memory_order_acquire) == 0)
757 ✗ return 0;
758
759 ✗ if (timeout_us == 0)
760 ✗ return 0;
761
762 ✗ ++idle_thread_count_;
763 ✗ if (timeout_us < 0)
764 ✗ wakeup_event_.wait(lock);
765 else
766 ✗ wakeup_event_.wait_for(lock, std::chrono::microseconds(timeout_us));
767 ✗ --idle_thread_count_;
768 129804 }
769 242373 }
770
771 } // namespace boost::corosio::detail
772
773 #endif // BOOST_COROSIO_HAS_SELECT
774